RestEasy reference guide :
if the application jar contains an Application class (or a subclass thereof) which is
annotated with an ApplicationPath annotation, a web.xml file isn't even needed.
If there is an Application class but it doesn't have an @ApplicationPath annotation,
then a web.xml file with at least a element is required.

Resteasy uses the ServletContainerInitializer integration interface in Servlet 3.0
containers to initialize an application, automatically scanning for resources
and providers. To enable automatic scanning, you must also
include the resteasy-servlet-initializer
//Resteasy reference guide